Programmable Logic Controllers (PLC's) are being used more and more frequently as their cost and size decrease. This increased usage is also due to increased functionality while at the same time a dramatic increase in the number and types of operating systems which a PLC may cooperate with as well as software tools which greatly facilitate the writing of appropriate instruction sets.
Accordingly, PLC's once installed are likely to be modified or adapted on the fly with, for example, new operating instructions, additional input/output (I/O) instructions and signals, particularly instrumentation. As a result of this PLC's are being used in many more applications.
Although many types of inputs are commonly connected to PLC's, on of the most frequent is from analog transducers or instruments, the most frequent typically being thermocouples used for temperature measurements. However, also increasing in usage are analog transducer inputs from devices such as tank gauges. However, what is common is that these types of devices are analog and, particularly in the use of thermocouples produce or dissipate heat. Moreover, since these inputs are analog, anything which affects the transducer input, or adjacent transducer inputs may result or inaccuracy. Therefore, this inaccuracy must either be accepted or compensated for so that the program in the PLC will properly operate.
It is generally known that acceptance of inaccuracies is only acceptable in only the most limited of situations. Therefore, designers are left with the solution of pre-determining what those inaccuracies and compensating for them.
Another problem associated with PLC inputs is that users typically have a choice of digital or analog inputs which are not definable but are hardware driven. While this typically does not present a problem upon initial design and implementation, it is problematic when instrumentation is changed or added. There, the user must condition the instrumentation at its point of use since, that is an analog signal must be digitized if only digital inputs are available on the PLC. Similarly, the reverse occurs where PLC outputs are concerned.
Also, due to packaging constraints, particularly among the newer "brick" size PLC's, the connector for input/output (I/O) connections occupies the entire front usable space of each module. This leaves little or no space for various other items such as logic connector switches, potentiometers, fuses or operator interface items. This size problem is particularly acute due to thermal variations between the upper and lower portions of the connector due to thermal transfer between the connector contacts and the module or PLC. It is known that in order to accurately design thermocouple PLC modules, the reference thermocouple wires must be held at relatively the same temperature throughout its length in order to maintain a consistent and stable junction temperature. Therefore, crowding of thermocouple wires or the simple intermixing of simple analog wires with thermocouple wires frequently results in very poor thermal characteristics which give upper contacts much higher temperature readings than lower contacts.
Accordingly, it is desirable and an object of the present invention to produce a device which minimizes thermal gradients between different portions of the I/O contacts.
It is also desirable and an object of the present invention to produce a device which allows for the use of additional components adjacent the PLC such as, potentiometers, switches, displays, other types of connectors and the like.
Still a further object of the present invention is to produce a device which allows for signal conditioning into or out of the PLC.
Finally, it is yet another object of the present invention to produce a connector for use with a Programmable Logic Controller (PLC), comprising an interface connector board consisting of a plurality of conductive paths thereon, the interface connector board having an edge and a planar surface disposed at an angle with respect to the edge; a plurality of conductive contact fingers which are at a spacing which is different than that of said conductive paths, each of which being connected to at least one of said plurality of conductive paths, the plurality of conductive fingers disposed along the edge of the interface connector board; and at least one secondary interface device disposed on the planar surface, the secondary interface device electrically connected to at least one of said plurality of conductive paths contained on the interface connector board.